Permanent-split capacitor (PSC) motors are widely used in a number of applications including HVAC fan and pump applications less than 5 hp. PSC motors generally operate at efficiencies from 20% to 65%. It is estimated that 100 million PSC motors are currently installed in HVAC applications in the United States, operating at average nominal efficiencies near 50%. These low efficiencies represent a significant cost, whether measured in equivalent pounds of carbon emissions, dollars, or other metrics.
FIG. 8 illustrates a conventional PSC motor system 900, including a power supply 902, a PSC motor 950, and a tap selector 960. Power supply 902 has a first terminal connected to a line rail 991 and a second terminal connected to a neutral rail 992. Motor 950 has a main winding 952, an auxiliary winding 954, a rotor (not shown), and a capacitor 959. Main winding 952 and auxiliary winding 954 are connected to neutral rail 992 by a common neutral pole 965. Auxiliary winding 954 is connected to line rail 991 by auxiliary pole 964. Main winding 952 has multiple taps 961, 962, 963, each connected to main winding 952 at a different location of the winding. Tap selector 960 connects line rail 991 to main winding 952 via one of the taps. Motor 950 is operable at three discrete speeds, with the motor speed being determined by the tap to which line rail 991 is connected via tap selector 960. The permanently installed capacitor trades off starting torque capability at standstill with ripple torque reduction at running speed. Due to the high volt-ampere (VA) rating of capacitors, the capacitor is often selected to meet the minimum starting performance requirements, resulting in poor running efficiency.
The main winding in a PSC motor is traditionally designed to meet a specified breakdown torque performance. The auxiliary winding is designed for a specific range of values of capacitors to limit inrush current that meet single-phase utility connection requirements, to provide adequate starting performance, and to meet the physical constraint of fitting the conductors in the slot area. The balancing of these tradeoffs does not lead to the best machine efficiency, particularly at small power ratings. Consequently, small size wire and inexpensive materials are used, resulting in significant winding resistances and core losses. These drawbacks have led many to abandon PSC motors in favor of three-phase motors driven by a three phase inverter. However, retrofitting an existing system in this manner requires the replacement of the motor and the introduction of a power electronic system to realize the upgrade. Historically, it has been prohibitively costly to install a full scale three phase variable speed drive with a motor in HVAC applications.
Main winding 952 and auxiliary winding 954 require different currents for proper operation of the motor. While the voltage applied to the main winding need not be more than the utility supply voltage, the voltage applied to the auxiliary winding needs to be sufficient to balance the machine's magneto-motive force (MMF). These constraints require a DC bus voltage at least greater than about 1.5 times the peak value of the rated voltage of the motor when a conventional inverter is used to retrofit a PSC motor. Such high voltages can pose a risk of partial discharge events resulting in damage to the windings. In other words, conventional inverters can realize the full voltage requirement for auxiliary winding 954 only at the cost of increased voltage stress on main winding 952. There is thus a need for the unique and inventive systems and methods disclosed herein.